This invention relates to laser diodes and more particularly to laser diodes which are fabricated in a manner to permit visual inspection during fabrication.
High power edge emitting laser diodes are traditionally bonded, with the diode junction side down, onto a high thermal conductivity heatsink using solder. Once the devices are bonded down, visual inspection of the facets of the diodes is done. If a small void is seen under the emitting region at the diode-solder interface the device is usually rejected as there is higher thermal resistance at that location and the device is prone to early failure. The device is then tested for electrical and optical performance. If the device passes certain minimum requirements the device is usually put into burn-in for a fixed amount of time. Once the burn-in is completed, the degradation of device performance determines if the device would pass or fail.
The burn-in step helps in the elimination of infant failures. As the device ages under normal operating conditions, failures occur due to gradual degradation of the device crystal structure. This is due to defects in the crystal lattice like Dark Line Defect or Dark Spot Defect. These defects occur due to lattice mismatch or point defects or mechanical damage like a scratch on the diode surface. These types of defects may not show their existence during the normal burn-in screening procedure. Once the device is bonded to the heatsink there is no easy way to look at the junction side surface of the chip as it is embedded in the solder.
The existence of lattice defects has been investigated by various ways of which one was to look at the spontaneous emission coming out from the substrate side of the device. This has been done for surface emitting devices, where the laser emission occurs from the substrate side, by creating a window in the metal contact (usually n-contact). For edge emitting devices it is achieved by selectively removing the metallization on the substrate side (usually n-contact) opposite the contact region (p-side) which is bonded to the heatsink. This procedure of selective metallization removal for investigating the failure mechanism on an edge emitting device is done after the device has failed.
In accordance with the principles of this invention screening techniques for infant failure mechanisms are improved by making use of the transparency properties of the substrate to the lasing or spontaneous emission. Specifically, an edge emitting device is fabricated with a window in the n-metallization region so that the optical characteristics all along the device cavity length can be compared before and after burn-in. Many lattice defects are more apparent after the device is subjected to electrical, thermal and optical stress. As these defects are regions of non-radiative recombination they show up as dark spots when the device is viewed with a camera sensitive to the emission wavelength. If a device is seen with these defects it may be rejected as the device is prone to early failure. This technique can be used to successfully eliminate device that may fail early in the field and result with product that has very high reliability. This technique can also reduce the time of burn-in required for very high reliability applications such as space communication and telecom where burn-in times sometimes in excess of one-thousand hours may be required.